BULLETIN 750, U. S. DEPARTMENT OF AGRICULTURE. 

 PREPARATION OF CALCIUM ARSENATE. 



The Bureau of Entomology of this Departmenl has carried out 

 experiments with "homemade" calcium arsenate prepared by slaking 

 burned lime with a solution of sodium arsenate. Others 3 have 

 recommended calcium arsenate prepared by the action of soluble 

 Lime sails on sodium arsenate. In all such cases soluble by- 

 products are formed. Since these may influence the action of the 

 spray if not removed, they should be washed and filtered out if a 

 good grade of commercial calcium arsenate is desired. 



From the chemical standpoint, the simplest means of producing 

 calcium arsenate is the direct union of calcium hydroxid and 

 arsenic acid. As the only by-product of this read ion is water. 

 filtration of the material is the only treatment thai should be re- 

 quired, thus eliminating the washing necessary with most products. 

 The following experiments were made with the idea of determining 

 the proportions and procedure necessary to obtain a product hav- 

 ing the properties desired for spraying purposes-. The arsenic acid 

 used was commercial 75 per cent acid. Its density Mas found to 

 he 1.88, corresponding to 01 per cent As 2 0-. or 75 per cent arsenic 

 acid, H 3 As0 4 . The lime was a good grade of commercial lime of 

 the following composition : 



Per cent. 



Loss on ignition - i3.07 



Calcium oxid, CaO >. 93.36 



Magnesium oxid. Mg< > GO 



Silicon dioxid, Si0 2 54 



Oxids of iron and aluminum, R 2 3 68 



100.34 



In the absence of other drawbacks, the most desirable method of 

 uniting lime aod arsenic acid is to have both ingredients in solu- 

 tion before they are mixed. This necessitates the use of lime water 

 which contains only aboul I gram of CaO per liter, ami of a quan- 

 tity of \\;iicr so huge that it, probably would be prohibitive in com- 

 mercial practice. Nevertheless, a few experiments were made to 

 discover the nature of the product so formed. 



1 Oregon Agr, Exp. Sta, Btfl. 131. 



deferable features of .•: good arsenical spray material maj bi enumerated here. 

 It should uol contain an excessive amount of soluble arsenic, which would cause 

 injury to foliage; it should be very finely divided in order that it may he dis 

 tributed well over the foliage; il should ho light when in the dry form, to permit 

 <.i ready application by dusting; il should suspend well in water, to permit of 

 even distribution when applied in the form of ;i spray; and when sprayed mid 



equentlj dried ii should stick «'oii 10 i ho foliage, 



